The present invention relates to a time division multiple access mobile wireless telecommunication system.
Conventionally, as described in a publication entitled "PERSONAL HANDY PHONE SYSTEM, RCR STANDARD (PROPOSAL), VERSION 1" edited by Research & Development Center for Radio System and issued Mar. 17th 1993, particular slots in a frame are only used to transmit information.
If a variety of wireless telecommunication systems are simultaneously operated in the same communication area in the same frequency band, the systems possibly interfere with each other in communications. For this reason, in order to permit a variety of wireless telecommunication systems to operate in the same communication area in the same frequency band, it is necessary to confirm, before a terminal belonging to one of the systems transmits a radio wave, whether or not the frequency of the radio wave is currently being utilized by any other system for communications in the area. This confirmation action is called carrier sense. After confirming that the frequency is still available by performing the carrier sense, communications can be started. It is described in FCC (Federal Communications Commission) "PUBLIC NOTICE, DA 93-1278", issued Oct. 22, 1993, .sctn.15,321 (c)-(1) that isochronous device is required to confirm the above for a period of at least 10 msec. prior to initiating transmission.
However, even if a wireless telecommunication system starts communications after the availability of the frequency has been confirmed, this wireless telecommunication system could be interfered by an interrupt of communication signals generated by another wireless telecommunication system which would start communications in the same frequency band after the former wireless telecommunication system has started the communications.
Even if the former wireless telecommunication system continues the communications while continuously performing the carrier sense after it has started the communications in order to avoid such interference, since the carrier sense is performed only for a short time of approximately 10 ms, a wireless telecommunication system which transmits information with a frame of a longer time than the carrier sense time may admit interrupts by other wireless telecommunication systems in the same frequency band in the course of its communications. Consequently, the interference similar to the above cannot be prevented even if the carrier sense is performed. Thus, disadvantageously, it is necessary to previously establish countermeasures to be taken in case an interrupt by another wireless telecommunication system into the same frequency is found.
Also, in a wireless telecommunication system which provides a plurality of cells, i.e., communication areas in each of which personal stations are allowed to communicate with a base station installed therein, a common control signal for commonly controlling the plurality of cells must be transmitted at the same frequency but in different time slots among these cells, such that the common control signal can be easily and rapidly captured even when a personal station moves from a cell to another. However, if this common control signal frequency chanel has already been occupied by another wireless telecommunication system in a cell, the wireless telecommunication systems cannot at all utilize the frequency.
Assuming that a terminal in a wireless telecommunication system performs the carrier sense over a time corresponding to the length of a frame defined in the system, if the frame is relatively long, the probability that a frequency, which is intended to be utilized in any of time slots within the frame, has already been utilized by another wireless telecommunication system becomes extremely high, resulting in a fear that a frequency channel required for the wireless telecommunication system itself cannot be captured.